The kinetics of binding of the thyroid hormones, L-thyroxine (L-T,) and ~-3,5,3'-triiodothyronine (L-T~), to human serum prealbumin were measured by a rapid gel filtration procedure to separate protein-bound from free hormone. The association rate constant for the ~-T,-prealbumin complex is comparable in magnitude to that of the ~-T~-r eceptor complex. A lower limit for the ~-T~-prealbumin association rate constant is lo6 M-' min-'. The dissociation rate constant for the ~-T,-prealburnin complex is higher than that for the ~-T,-prealbumin complex. This is attributed to steric effects, due to the extra iodine atom of L-T,, that are operative in L-T, dissociation from prealbumin. The dissociation rate constant for the ~-T,-prealbumin complex is much higher than that for the ~-T~-receptor complex, implying that the receptor binding site is in some sense more confined than the binding site of prealbumin.

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